An input shaft of a transmission may be tied to a case of a transmission during engine starting. Tying the transmission input shaft to the transmission case allows the transmission to supply an increased load to the engine as compared to when the engine is in neutral so that engine speed may be controlled during an engine start. Additionally, less engine torque may be transmitted to vehicle wheels when the input shaft is coupled to the transmission case instead of vehicle wheels. In this way, a load may be applied to an engine to control engine speed during starting while at the same time limiting torque to accelerate the vehicle to which the engine is coupled. However, if one or more clutches tying up the transmission is supplied oil at a low pressure, the clutch may slip and cause a torque disturbance in the vehicle driveline. Alternatively, if the clutch is supplied oil at a higher pressure, it may take a longer amount of time to release the clutch so that vehicle launch may be delayed. Further, transmission temperatures as well as changes in road grade can affect whether or not a transmission tie-up clutch holds the transmission input shaft in a desired manner. Therefore, a single transmission clutch tie-up oil pressure command may not provide desirable tie-up characteristics during varying vehicle operating conditions.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for controlling a transmission coupled to an engine during an engine start, comprising: applying a first pressure to a clutch tying a transmission input shaft to a transmission case during a first engine start; and applying a second pressure to the clutch tying the transmission input shaft to the transmission case during a second engine start.
By supplying different pressures to one or more clutches tying up a transmission input shaft to a transmission case, it may be possible to provide desirable tie-up forces to couple a transmission input shaft to a transmission case during a variety of operating conditions. For example, for a transmission that includes an over-running clutch, oil pressure supplying force to a tie-up clutch (e.g., a gear clutch other than first gear clutch) can be adjusted such that the tie-up clutch force coupling the transmission input shaft to the transmission case is different for different engine and/or transmission operating conditions. During some operating conditions, oil pressure supplied to the tie-up clutch may be held at a higher pressure than during other operating conditions. For example, when a vehicle to which the transmission is coupled is stopped on a road with a grade, the road grade may be compensated for by increasing oil pressure supplied to the tie-up clutch. Similarly, during some operating conditions, oil pressure supplied to a tie-up clutch may be reduced during conditions where it takes more time than is desired to relieve the oil pressure and release the tie-up clutch. Further, the oil pressure supplied to a tie-up clutch may be adaptively changed so that different transmissions of different vehicles may tie-up and release in a similar manner even though there may be differences between transmission components.
The present description may provide several advantages. For example, the approach may reduce vehicle launch variability for engines that are automatically stopped and started. Further, the method may reduce transmission clutch wear since transmission tie-up application and release can be adjusted for operating conditions. Further still, engine emissions may be reduced as compared to systems where engine speed varies due to slipping of the transmission during transmission tie-up.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.